1. Field of the Invention
The present invention relates in general to a digital processing device provided with plural types or varieties of memories. In particular, the invention concerns an improvement in digital data processing devices in which the processing of a data transfer with plural types of memories can be accomplished at an increased speed.
2. Description of the Prior Art
In recent years, remarkable progress has been made in high-density integrated semiconductor circuit technology and there is now available commercially inexpensive large scale integrated circuits (LSIs) for electronic computers which incorporate a central micro-processing unit (hereinafter referred to as a MPU in abridgement) integrated on a single chip. Under the circumstances, an electronic computer of a very small size which is constituted by a MPU provided additionally with input/output devices and memory or memories for independent operation tend to be increasingly and widely used particularly for personal use or business applications of small scale. These electronic computers are usually equipped with a keyboard for the input device and a cathode ray tube display (hereinafter referred to as a CRT) for the output device. Information or data input through the keyboard as well as characters, graphs, charts or the like graphic patterns which are composed with the aid of programs are displayed on the display screen of the CRT. In view of the processing capability of these small scale computers and the manufacturing costs for the memories or the like, there is used dominantly the MPU which is capable of processing 8-bit data in parallel. In this connection, it is also known that the display screen of a CRT is divided into a large number of areas or sections, whereby the characters or graphs are displayed in color by allotting predetermined colors to the divided sections in the patterns corresponding to the characters or graphs to be displayed. In such a case, the display on a single area or section requires as many as sixteen bits in total of which eight bits are used for designating the type of alphabet or numeral to be displayed in that area or division, while the other eight bits are used for designating the color of the alphanumeric character and the background color displayed in the same divided area.
In order for the MPU of the eight-bit processing capability to be able to process the data of sixteen bits, a bank switching system may be adopted in combination with a plurality of memories for exchangeably selecting one of the memories for performing data transfer with MPU. In more particular, each data item of sixteen bits is divided into a character code of eight bits and a color code of eight bits, both codes then being separately stored in a character memory and a color memory, respectively. Under these conditions, the MPU performs alternate selection of one of the two memories and then effects the data transfer with the selected memory for every display division or area in a sequential manner.
By the way, in the case of a graphic display, it is often required that the contents to be displayed make an appearance in a so-called scrolling manner (e.g. a graph is displayed following a rising-up trace). In such case, the concerned data stored at a certain address in the memory has to be shifted from the certain address to another proper address which is spaced from the former by a predetermined number of addresses. Such being the circumstances, the data transfer executed for the data of sixteen bits for every display section or division requires a large number of processing steps including the operations of selecting a character memory, reading contents from the character memory at a given address, writing the read-out character code at the address distanced from the given address by a predetermined number of addresses, selecting a color memory, reading the contents from the color memory at a given address, and writing the read color data at an address spaced from the given address by a predetermined number of addresses. Such a data transaction will necessarily involve an intolerable time consumption.
Further, transient states during the transaction or transfer operations will disadvantageously be perceptible to the eye and the ear of the user or operator. Besides, the scrolling display cannot be produced in a natural manner.